In recent years, improvement in fuel economy (fuel consumption) has been required in relation to enhancement of exhaust gas regulation for carbon dioxide, or depletion of fossil fuels. As effective countermeasures against these concerns, great attention is paid to a downsized engine, the size of which is decreased by reducing engine displacement, and output is obtained using a supercharger. In the downsized engine, pumping loss or friction is reduced due to the reduction of the engine displacement, and thus it is possible to improve fuel economy. In contrast, it is possible to improve fuel economy by obtaining sufficient output using the supercharger, and preventing the lowering of a compression ratio associated with supercharging by virtue of intake air cooling effects associated with direct in-cylinder injection. In particular, the fuel injection device used in the downsized engine is required to be able to inject fuel over a wide range from the minimum amount of injection corresponding to the minimum output associated with small engine displacement to the maximum amount of injection corresponding to the maximum output associated with supercharging, and the expansion of a control range of the amount of injection has been required.
Typically, the amount of injection of the fuel injection device is controlled by a pulse width of an injection pulse that is output from an electronic control unit (ECU). When the injection pulse width is increased, the amount of injection is increased, and when the injection pulse width is decreased, the amount of injection is decreased. The relationship between the injection pulse width and the amount of injection is substantially linear. However, in the region with a small injection pulse width, due to a rebound phenomenon (rebound motion of the movable core) occurring when the movable core comes into contact with the fixed core and the like, the time from when the injection pulse is stopped and to when the movable core reaches a closed valve position is changed, and since the amount of injection is not linearly changed relative to the injection pulse width, the controllable minimum amount of injection of the fuel injection device is increased, which is a problem. The amounts of injection of individual fuel injection devices may not be stable due to the rebound phenomenon of the movable core, and thus the controllable minimum amount of injection has to be set based on an individual fuel injection device with the maximum amount of injection, thereby causing an increase in the controllable minimum amount of injection. When the injection pulse width is further decreased from a non-linear region in which the relationship between the injection pulse and the amount of injection is not linear, the movable core does not come into contact with the fixed core, that is, the movable core is present in a medium lift region in which the valve body is not fully lifted. In the medium lift region, even if the same injection pulse is supplied to the fuel injection device for each cylinder, the amounts of lift of the fuel injection devices are different due to the difference between the individual fuel injection devices caused by dimensional tolerances of the fuel injection devices, and thus individual-to-individual variations in the amount of injection are increased, and the driving of the fuel injection device in the medium lift region becomes a problem from the viewpoint of combustion stability.
As described above, in order to improve fuel economy, it is necessary to reduce variations in the amount of injection of the fuel injection device and to reduce the controllable minimum amount of injection, and in order to considerably reduce the minimum amount of injection, it is required to control the amount of injection in the region with a small injection pulse in which the relationship between the injection pulse width and the amount of injection is not linear, or the medium lift region in which the injection pulse is small, and the valve body does not reach a target amount of lift.
In order to reduce variations in the amount of injection and the minimum amount of injection, the drive unit for the fuel injection device for each cylinder is required to be able to detect changes (which are caused by a rebound phenomenon occurring when the movable core comes into contact with the fixed core and the like during valve opening) in the time from when the injection pulse is stopped and to when the movable core reaches a closed valve position, variations in valve operation, or variations in the amount of injection.
A fuel injection control device disclosed in PTL 1 detects a timing when the movable core comes into contact with the fixed core by detecting a timing when a second-order differential value of current switches from a negative value to a positive value based on a phenomenon in which magnetic resistance of a magnetic circuit (which is formed by the movable core and the fixed core) is reduced due to a rapid decrease in the air gap between the movable core and the fixed core, and the magnetic materials are magnetically saturated and inductance in the magnetic circuit is changed due to an increase in magnetic fluxes through the movable core and the fixed core.
According to a method disclosed in PTL 2, based on the fact that the on and off cycle of the drive current of the electromagnetic valve increases when a valve opening operation progresses, and inductance in a drive coil increases, the valve is determined to be opened when the on and off cycle is longer than a set value.